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Boris Nemtsov, pictured on the banner, came to prominence under former Russian President Boris Yeltsin

Russian investigators say a Chechen security officer ordered the murder of opposition politician Boris Nemtsov, but he is believed to have fled abroad.

Mr Nemtsov, a prominent critic of President Vladimir Putin, was shot dead in central Moscow in February.

Russia's Investigative Committee (SK) is set to formally charge five Chechens - currently under arrest - with carrying out the "contract killing".

But Ruslan Mukhudinov - the suspected organiser - is still at large.

In a statement (in Russian), the SK said a separate indictment would be drawn up against Mr Mukhudinov "and other, as yet unidentified figures". An international arrest warrant was issued for Mr Mukhudinov last month.

Mr Mukhudinov served in the Chechen "Sever" (North) Battalion as the driver for Ruslan Geremeyev, an associate of Chechen President Ramzan Kadyrov.

According to Vadim Prokhorov, a lawyer for Mr Nemtsov's family, the investigators have exaggerated Mr Mukhudinov's role, and "the masterminds are high-ranking people".

"Mukhudinov hasn't been caught, or questioned, his whereabouts are unknown, so you cannot talk about him being the one who ordered the killing," said Mr Prokhorov, quoted by Interfax news agency.

Kadyrov associates

Mr Nemtsov - once a Russian minister - was shot on the night of 27 February on a bridge near the Kremlin.

The liberal reformer had sharply criticised high-level corruption in President Putin's Russia and the Kremlin's role in the Ukraine conflict.

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Five suspects have appeared in court - but the alleged mastermind is at large

In October the Moscow court handling the murder case rejected a request from Mr Nemtsov's daughter, Zhanna Nemtsova, for President Kadyrov to be questioned.

The Chechen president has run the Russian republic in the North Caucasus with an iron fist, enforcing Russian control there against separatist rebels and other opponents.

The alleged gunman in the Nemtsov case, Zaur Dadayev, was previously deputy commander of the North Battalion and was another ally of Mr Kadyrov.

In March, after Mr Dadayev's arrest, Mr Kadyrov wrote in defence of him, saying he was "sincerely devoted to Russia, ready to give his life for the motherland".

But he added that Mr Dadayev had been angered by French satirical magazine Charlie Hebdo's cartoons of the Prophet Muhammad. It has been suggested that the murder may have been a reprisal for comments that Boris Nemtsov made in support of Charlie Hebdo employees.